The present invention generally relates to echo cancellers, and more particularly to a sub-band acoustic echo canceller which is applicable to a video/audio conference communication systems, long-distance communication systems and the like.
In long-distance communication systems such as satellite communication and submarine cable communication systems, an echo deteriorates the quality of the call. This echo occurs when a signal which is received from a calling station at a receiving station returns to the calling station with a transmission signal from the receiving station due to a mismatch of a hybrid transformer in a two-wire/four-wire converter part. On the other hand, in video/audio conference systems and loudspeaker telephone sets, the sound output from a speaker is reflected by walls of the room or the like and mixes in as an input to a microphone thereby generating an echo sound.
An echo canceller is used to cancel the above described echo. But in the video/audio conference system, for example, the impulse response of the system from the speaker to the microphone becomes extremely long. As a result, the number of tap coefficients required becomes extremely large when the normal finite impulse response (FIR) type echo canceller is used, and the scale of the hardware becomes extremely large.
In order to solve the above described problem, a sub-band acoustic echo canceller has been proposed. Although the scale of the hardware of this sub-band acoustic echo canceller is small, the quality of the call after the echo cancellation is poorer compared to the general echo canceller and there is a demand to realize a sub-band acoustic echo canceller having an improved performance.
FIG. 1 shows an example of a conventional sub-band acoustic echo canceller. This echo canceller is applied to a video/audio conference system, for example, and an audio signal received from a line is output from a speaker 8 while an audio signal input from a microphone 9 is transmitted to a line.
In FIG. 1, a division and decimation process part 10 includes a filter bank 101 and a decimation part 102. The filter bank 101 is made up of quadrature mirror filters (QMFs) and divides a received signal into N band signals in corresponding bands of N channels CH1 through CHN. The band signals in the respective bands are subjected to a 1/N decimation in the decimation part 102 before being supplied to an echo canceller group 40. The 1/N decimation is a process in which one sample is successively extracted from N samples.
A division and decimation process part 20 has the same structure as the division and decimation process part 10, and includes a filter bank 201 and a decimation part 202. The N band signals from the decimation part 202 are supplied to the echo canceller group 40.
The echo canceller group 40 is made up of a group of echo cancellers for cancelling the echo in each of the bands. For example, the echo canceller of the channel CH1 includes an adaptive digital filter (ADF) 401.sub.l for generating a pseudo echo based on a band signal CH1 from the division and decimation process part 10, and a subtractor 402.sub.l for generating a residual signal (residual echo) by subtracting the pseudo echo from a band signal CH1 from the division and decimation process part 20. This residual signal is used for controlling the renewal of tap coefficients of the ADF 401.sub.1 and is supplied to an interpolation and synthesis process part 30. The echo cancellers of the other channels CH2 through CHN have constructions identical to that of the echo canceller of the channel CH1.
The interpolation and synthesis process part 30 includes an interpolation part 301 and a synthesis filter 302. The interpolation part 301 carries out an interpolation process in which the signals of each of the channels subjected to the 1/N decimation in the decimation parts 102 and 202 are restored into original signals. According to this interpolation process, a zero sample is inserted into each of the decimated signals at a rate of 1 in every N-1 samples. The synthesis filter 302 adds the interpolated band signals and generates original transmission signals which are transmitted to the line.
FIG. 2A shows a filter characteristic of the filter banks 101 and 201 of the respective division and decimation process parts 10 and 20. As shown in FIG. 2A, the input signal is divided into N band signals of the channels CH1 through CHN by the filter bank 101 or 201 which is made up of complex filters. In FIG. 2A and FIGS. 2B through 2E which will be described later, fs denotes a sampling frequency.
Each band signal is subjected to the decimation in the decimation part 102 or 202. In this case, the filter characteristic after the decimation for the odd channels CH1, CH3, CH5, . . . becomes as shown in FIG. 2B, while the filter characteristic after the decimation for the even channels CH2, CH4, CH6, . . . becomes as shown in FIG. 2C.
FIG. 2D shows a signal which is obtained by taking a real part after the band signals of the odd channels pass through the division and decimation process part 10 or 20. Similarly, FIG. 2E shows a signal which is obtained by taking a real part after the band signals of the even channels pass through the division and decimation process part 10 or 20. These signals shown in FIGS. 2D and 2E include aliasing components. In FIGS. 2D and 2E, an arrow pointing right indicates the upper side band of the signal while an arrow pointing left indicates the lower side band of the signal, and the lower side band appears as the aliasing component. The real part signal shown in FIGS. 2D and 2E are used as output signals of the decimation parts 102 and 202, and the echo canceller group 40 operates responsive to the real part signals.
In each of the odd and even channels, the information quantity of each channel is reduced to 1/N by the 1/N decimation. For this reason, the echo canceller which is provided in a stage subsequent to the decimation part can reduce the number of tap coefficients.
A description will now be given of an operation of the conventional echo canceller shown in FIG. 2. A reception signal from the line is input to the division and decimation process part 10 wherein the reception signal is divided into N band signals CH1 through CHN and decimated. The output signals of the division and decimation process part 10 are input to the echo canceller group 40 wherein a pseudo echo of the echo which is mixed to a transmission signal due to the output of the speaker 8 picked up by the microphone 9 is generated in each of the ADFs 401.sub.l through 401.sub.n. The pseudo echo is subtracted from the corresponding band signal of the transmission signal which is processed in the division and decimation process part 20 in one of the subtractors 402.sub.l through 402.sub.n, and the residual signal of each channel is output from the echo canceller group 40.
Each residual signal is interpolated in the interpolation part 301 of the interpolation and synthesis process part 30 in the corresponding one of the channels CH1 through CHN. The interpolated residual signals of the channels CH1 through CHN output from the interpolation part 301 are added in the synthesis filter 302 of the interpolation and synthesis process part 30 and restored to the original residual signal having all of the frequency bands. The output signal of the synthesis filter 302 is supplied to the line.
When compared to the FIR type echo canceller having the direct form, the signal processing quantity of the conventional sub-band acoustic echo canceller described above is approximately 1/N because the sampling rate of the signal after the decimation is 1/N that of the FIR type echo canceller preserving the total number of ADF taps same as the FIR type echo canceller. As a result, it is possible to reduce the scale of the hardware according to the conventional sub-band acoustic echo canceller.
In the conventional sub-band acoustic echo canceller, the echo cancelling process in the echo canceller group 40 is carried out with respect to the real part components of the signals output from the division and decimation process parts 10 and 20. As may be seen from FIGS. 2D and 2E, the real part components have overlapping parts between the band signals due to the aliasing component. When this overlapping part is generated, it is impossible to sufficiently suppress the error between the bands of the residual signal after the synthesis in the interpolation and synthesis process part 30. As a result, there is a problem in that the echo suppression quantity as a whole becomes small.
FIG. 3 shows a spectrum of the residual signal obtained in the conventional sub-band acoustic echo canceller for explaining the effects of the error between the bands. In FIG. 3, the abscissa indicates the frequency and the ordinate indicates the signal level. A solid line I indicates the spectrum characteristic of the residual signal and a dotted line II indicates the spectrum characteristic of the transmission signal when no echo cancellation is carried out. As may be seen from FIG. 3, the error suppression characteristic deteriorates at the boundary of the bands due to the overlapping part between the band signals.
For example, this problem is discussed in Andre Gillorie, "Experiments with Sub-Band Acoustic Echo Cancellers for Teleconferencing", ICASSP '87, 49.12.1, pp. 2141-2144.
On the other hand, the interpolation and synthesis process part 30 carries out the interpolation and synthesis with respect to the residual echo of each band output from the echo canceller group 40, but the residual echo is sufficiently small when the echo canceller is operating normally. For this reason, if the interpolation and synthesis process part 30 is designed to make a fixed-point operation, it becomes impossible to obtain a sufficient dynamic range with respect to the residual echo and there is a problem in that the echo suppression characteristic deteriorates due to the effects of the operation accuracy.
In addition, in the conventional sub-band acoustic echo canceller, the transmission signal input from the microphone 9 is transmitted to the line via the division and decimation process part 20, the echo canceller group 40 and the interpolation and synthesis process part 30. Consequently, the following problem is generated.
That is, the order of the filter banks used in the division and decimation process part 20 and the interpolation and synthesis process part 30 is finite. As a result, a ripple is introduced to the signal at the filter bank and a spectrum of the transmission signal after the synthesis becomes distorted.